As surfing has increased in popularity to the point that international competition is now prevalent, various refinements and structural and design changes in surfboards have been made in an effort to provide optimum effectiveness in the surfing operation. As obvious examples, the overall length of surfboards has decreased in recent years, and it is now common to mold surfboards from polyurethane foam with longitudinally extending bars or stringers for reinforcement. Experienced surfers have also found that the precise shape and/or disposition of the control fin on the undersurface of the surfboard are critical and attempts have been made to provide a removable and/or adjustable fin arrangement enabling accommodation of the many variables encountered such as the size and weight of the particular surfer, the types of waves encountered, and the size and weight of the surfboard itself. While such adjustable or removable fin system is theoretically advantageous, certain practical difficulties have been encountered with the proposed arrangements. In the first place, it being recognized that the fin provides lateral stability in the control of the surfboard, it is essential that no fin base wobble in relation to the surfboard be introduced and such, regretably, has not been achieved. Furthermore, the most common adjustable units now in use employ a box support for the fin which is inserted into the body of the surfboard in the desired position and accordingly requires a cutting away of the central reinforcing bar or stringer so as to weaken the strength of the surfboard itself, an obviously undesirable effect. Finally, all of the proposed and now utilized removable or adjustable fin units in addition to the mentioned wobble, also introduce additional hydrodynamic inefficiency in the form of their structural characteristics which produce excessive separation drag and gross turbulence. As one example, the mentioned box structure provides an elongated opening in the bottom of the board which produces a gross amount of such separation drag and turbulence, thus reducing the fin efficiency in its control function.